The materials of the invention are preferably such as to be effective when driven by a relatively low voltage, in particular a voltage up to the order of 110 volts, 110 volts being the maximum in practice which is considered to be reasonably safe as far as electrocution of human beings is concerned. It is envisaged that the materials in future developments may be used with higher driving voltages e.g. 240 volts, but for the purposes of clarity of description and from a practical point of view, when reference is made hereinafter to low voltage it is intended to mean a voltage up to the order of 110 volts.
Sheet structures which are electrically conductive and constitute resistance heating waxes are of course known and an example is described in GB Patent Specification No 2261822A; other structures include textiles impregnated/coated with a carbon slurry and carbon fibers woven into a conductive mat. But, our investigations lead us to the belief that such structures generally, unless they are designed for specific applications and are specially constructed, fail to give even heating characteristics across their area, lack strength and/or are ineffective when driven by relatively low voltages. Furthermore, they do not provide flexible sheet structures which are robust and can withstand aggressive handling and can operate in damp and corrosive environments.
The present invention at least in its preferred form in meeting these requirements provides a considerable advance in low voltage resistance heating technology.
A main aspect of the invention resides in that a textile fabric of a particular type is used as an electrically conductive resistance heating element. The particular fabric which has been identified in this invention is one which in particular is a fabric containing synthetic material fibers, and in which the fabric has been subjected to a high temperature treatment in order to render the fabric fire and flame resistant.
Thus, a fabric made of polymeric fiber and baked in stages by heat treatments at high temperatures for a predetermined time has been produced for utilization in the past in relatively high tech applications. The baking of the fabric has the effect of carbonization of the polymer which is a process of formation of carbon in the fibers from the basic hydrocarbon material. As explained, this material has been produced in the past for high tech applications and in particular has been used in the nose cones of guided missiles, the purpose of the fabric being to make the nose cone highly heat resistant. The materials have also been used in other space technology applications again for heat and flame resistance. A third application is for the utilization of this material in the field of the formation of flame resistant wall structures.
The material has not heretofore been used as an electrical conductor, and indeed prior to the making of the present invention it had not been discovered that the material had excellent electrical conductivity properties and low resistance enabling conducting of relatively high currents at low voltage. The material when baked is in the nature of a fabric of a weight and consistency which may be compared to a typical textile furniture covering fabric, but it will usually be grey or black in color due to the carbonization of the polymeric material even if the fabric was not of such a dark color prior to the heat treatment.
Attaching bus bar conductors to such fabric at spaced locations, followed by the application of an electric potential between the bus bars has shown by experimentation that the fabric heats up evenly across the entire area of same, and the fabric furthermore efficiently converts the flowing electricity into resistance heat, even when relatively small driving voltages are applied. The possibilities for the utilization of such a material are endless.
The particular material which we have tested is a polyacrylonitrile based material of woven construction, although other materials and other structures such as knitted and other felted structures may be adopted. The heat treatment of the material was carried out in stages and involved baking at temperatures of 221.degree. C. and 1000.degree. C. respectively. According to preferred features of the invention, the carbonized fabric is sandwiched between protective layers in order to produce a flexible heating element. The sandwiching between the protective layers may leave the edges of the fabric exposed or may be such as to ensure that the fabric is encapsulated by the layers, which preferably render the entire flexible element waterproof and electrically contained.
The protective layers may be applied as coherent sheets to opposite sides of the fabric sheet followed by a laminating process involving either heat and pressure or glue and pressure, or alternatively either or both of the outer layers of the sandwich may be applied by a coating process involving the application of liquid coating materials which subsequently set firm either naturally or by the application of heat. Pressure preferably is also applied when coating materials are used, so that the coating materials will be able to flow through the interstices of the warp and weft of the fabric, it being remembered that a woven fabric is the preferred embodiment of the invention.
Any suitable flexible covering materials may be adopted and some examples are given hereinafter.
It is preferred that the resulting element be a tough flexible sheet structure which can either be formed in pieces or in a long length suitable for cutting into sections depending upon the application to which the section is to be put.
Preferably, bus bar connectors may be applied to the fabric before the coating or laminating takes place so that the bus bars will also be insulated by the laminates or coatings.
In one example, a continuous web of the fabric is fed in the direction of its length, and conductor strips are applied to the edges at both sides of the fabric, by a suitable adhesive or other bonding medium. Conductive strips may also be applied at any longitudinal position across the web in order to achieve a final mat size and electrical resistance appropriate for its final end usage. Additionally, for particular circumstances, conductive strips may be applied transversely across the width of the fabric. Coating materials are applied downstream of the application of the conductors in order to cover the fabric and conductors, and heat and pressure are applied in order to cure the coating layers as appropriate. There therefore results a continuous conductive web in which the fabric and the conductors are sandwiched between insulating layers. This web can then be cut transversely into lengths depending upon the application involved, and for each length, the resistance between the conductors increases as the length becomes shorter, and decreases as the length becomes longer. Therefore, by utilizing the sections in any desired pattern, e.g. by electrically connecting the sections in series, so the resistance of the resulting assembly can be varied and therefore the heating effect can be varied. When separate sections are coupled together they may be connected by means of electrical crimp terminals which are crimped through the encapsulation onto the conductors, but in this case it is preferable to use sealing tapes in order to seal or encapsulate the crimp connectors. Other forms of electrical connection (rather than crimp terminals) may be used. Also, the raw edges of the sections of the flexible element which are created by cutting the continuous web may be sealed by appropriate sealing tape or the like; in some applications this may not be necessary.
Although, as has been indicated herein, a major aspect of the present invention resides in the utilization of the particular carbonized fabric as an electrical conductor, with or without the encapsulation, the use of the encapsulation and conductive fabric presents another aspect of the invention, and in this aspect the conductive fabric may be any conductive fabric. Encapsulation again may be by laminating or coating.
By way of explanation of the main aspect of the invention, reference is now made to the accompanying diagrammatic drawings, wherein;--